DESCRIPTION: (Applicant's Description) Both quantitative and qualitative differences exist between high and low LET radiation in their ability to induce genetic damage; high LET radiation has a greater mutagenic efficacy than low LET radiation. Because the high LET radiation causes a transmissible destabilization of mammalian chromosomes, the mutation efficacy exceeds that for a single hit, leading to the cascading effect of mutation formation. The cellular processes that bring this about are presently unknown. Genomic integrity in mammalian cells is maintained by a complex set of enzyme systems that ensure proper replication, distribution and functioning of the genetic material. In this application, the applicant proposes to study two of these systems to ascertain their contribution to chromosome destabilization that is observed after exposure of mammalian cells to high LET radiation. Recent studies have described a series of signal transduction pathways that co-ordinate genomic maintenance activities and cell cycle progression. Different types of DNA damage activate different cell cycle checkpoints. Although much work has been done with gamma radiation in human cells, the cellular response to high LET radiation is less well studied. In this application, the applicant will study several of the cell cycle checkpoints known in human cells and determine if high LET radiation activates checkpoint control in the Gl, S, G2 or M phases of the cell cycle. More recently, she has identified a set of activities that generates multiple rearrangements in the human genome. While tumor cells are highly proficient in an enzyme system that activates homologous recombination, this activity is inhibited in normal cells. In characterizing this activity she found it to have biochemical properties similar to those of the REC A protein in bacteria. She has documented that the putative REC A homologue, hRAD51 is involved in these reactions. The remaining Specific Aims in this application will address the effects of high energy LET radiation on this system in an attempt to predict its biological impact. She proposes to determine if high LET radiation can activate strand transferase activity and homologous recombination in normal human cells. She will determine if tumor progression alters the cellular response to high LET radiation. And finally, she will determine if cellular responses to high LET radiation are unique or are shared with that of low LET radiation.